In devices such as those kept operated at high temperatures for a long duration of time, and operated under harsh environments such as exposed to large temperature changes between operational states at high temperatures and idle states at low temperatures, represented by vehicle-borne power control devices (power devices), bonding portions are required to be able to keep high bonding strength over a long duration of time. None of the conventionally known bonding members has, however, been enough to satisfy such requirement.
For example, a SnAgCu-based bonding member (powdery solder material) disclosed in JP-A-2007-268569 cannot satisfy the above-described requirement at all.
As a technique of enhancing heat resistance or bonding strength of the bonding portion, there has been known a method of increasing the content of a refractory metal contained in the bonding member, or increasing the amount of an intermetallic compound to be formed. The bonding member with such high content of refractory metal, however, needs higher temperatures for bonding, and this may cause damages in substrates and electronic parts. Meanwhile, in the strategy of increasing the amount of intermetallic compound to be formed, a metal that should principally be allocated to diffusion in conjunction with a member to be bonded would be consumed to form the intermetallic compound, and this may make the bonding incomplete.
There is still another problem of reduced mechanical strength caused by Kirkendall void. The Kirkendall void is generated when atomic vacancies (lattice defects), generated as a result unbalanced mutual diffusion of metals, accumulate rather than annihilate. For an exemplary case of a Sn—Cu interface, vacancies accumulate at the interface between an intermetallic compound and Cu, since Sn diffuses only to a lesser extent than Cu does, to thereby form the Kirkendall void. The Kirkendall void may grow up to a larger void or crack, may degrade reliability and quality of the bonding portion or the three-dimensional structure, may further reduce the mechanical strength, and can even result in separation, breakage, chipping or the like.
Japanese patent No. 5517694 has disclosed a method of forming a Ni layer on a member to be bonded, allowing Cu6Sn5 to deposit or migrate thereon to form a barrier layer, thereby suppressing growth of a compound layer due to a reaction at the bonding interface, and associated formation of voids.
The deposition or migration of Cu6Sn5 to the bonding interface, however, takes a certain time, and also during that time the reaction-diffusion process proceeds. It is therefore not always possible to suppress the Kirkendall void from generating.